Centralizers have been in use for a long time. As their name suggests they have been used to center a tubular in a borehole. Most commonly, centralizers are used to position casing in the borehole as the cement is delivered into the annular space around the outside of the casing to set up and seal the casing in the bore hole. Centralizers have also been used as guides for sucker rods in downhole pumps. The centralizers have been made of metal and non-metallic materials such as thermoplastic polyamides, glass and mineral filled nylons and poly-tetra fluoro ethylene, also known as Teflon and injection molded polyurethane. These centralizers were made in hinged segments that could be clamped onto a tubular and in some applications the centralizers were formed right on to or slipped over the rod or tubular. Illustrative examples of the variations in prior centralizers are shown in U.S. Pat. Nos. 4,483,395; 4,088,185; 3,963,075; 2,611,664; 5,908,072; 6,102,118; and 6,283,205.
More recently, tubulars such as casing have been expanded downhole after cementing and sometimes without cementing. The centralizers used on casing for expansion have been metallic and have caused problems. Recently, one such problem has been reported in the March 2002 issue of Drilling Magazine on page 36. There a tempered steel arm of a centralizer had broken off and damaged the pipe to be expanded. The problem is that if the pipe to be expanded is scored prior to expansion, the stress is concentrated at that area and a fracture is likely upon expansion. There has been some recognition of this concern in the way the expandable tubulars are handled on the surface. Expandable tubulars are picked up with forklifts that have padded forks. The joints are packaged with wooden dividers to avoid contact with each other. Nylon slings are used to pick up joints one at a time onto the rig. Non-penetrating tongs are used to get a friction grip on the connections during makeup. Despite all these surface handling precautions, metallic centralizers have continued to be used. These centralizers are sufficiently rigid to increase the force required on the swage for expansion downhole. At times, the swage has stalled as the stroker has tried to advance it in the location of a relatively unyielding centralizer. Occasionally, the metallic centralizers used on casing to be expanded have had flexible strips break during run in and have scored the outer surface of the casing to the point that when the casing was expanded the stress concentration at the point of scoring initiated a fracture failure during expansion.
When cementing a tubular, centralizers have been used to allow the cement to work its way fully around the tubular. Without centralizers, particularly if there is a deviation in the wellbore, there was a risk that the tubular would lay up against the borehole wall on the low side undermining the benefit of the cement in trying to seal around the tubular.
Centralizers of various types have been used in the past. The most common centralizers comprise a plurality of spaced flexible strips that extend longitudinally between a pair of end rings. The centralizers are slipped over the end of the tubular on makeup or get clamped to the tubular due to a hinge connection in each of the two end rings. These centralizers are typically made of steel. The problem with these centralizers arises if there is to be any pipe expansion. Expanding tubulars has become a more widely used procedure and such centralizers have been known to cause high stress areas on the underlying tubular during expansion to the point where the tubular can split or crack. Attempts to improve on the metal centralizers described above by making them from a polymeric material have had mixed results. The problem there has been that they are not strong enough to hold their shape to the extent that their main purpose of centralizing is defeated. Also, their geometric dimensions do not lend themselves to be run through the previous casing string and yet still have enough standoff for suitable centralization.
Centralizing devices have also been used that are bulky and that have many moving parts. These designs are expensive, require inordinate maintenance, and are simply too large to be of use in many applications. Some examples are U.S. Pat. No. 2,874,783 and PCT Application WO 94/13928.
Another centralizing technique for cementing tubulars has been to use telescoping cylinders that can be pushed out when the tubular is in position. These cylinders had removable barriers to let flow go through them after extension. One of their uses was to centralize a tubular prior to cementing. Some illustrations of this type of centralizing. system can be found in U.S. Pat. Nos. 5,228,518; 5,346,016; 5,379,838; 5,224,556; and 5,165,478. None of these centralizers were used in combination with tubulars that were to be expanded. Yet, despite the use of extendable cylinders to centralize prior to cementing, in applications where the tubular was to be expanded after cementing there was either no attempt to cement in an inclined wellbore or centralizers that wrapped around the tubular and caused stress failures when expanded were used. As a result, poor cement distribution occurred that was compounded by the subsequent expansion of the tubular. What is needed and addressed by the present invention is an effective way to centralize a tubular before it is cemented and subsequently expanded even if it is disposed in a deviated bore.
Apart from the various deign described above, in the context of tubular to be expanded in deviated borehole after being cemented, rubber strips were used in a helical pattern that extended from a ring that was bonded to the outside of the tubular with the strips extending generally longitudinally. These designs did not substantially resist expansion as the bow spring centralizers but they were logistically complicated in that the tubulars had to be shipped to a shop capable of bonding the rubber strips and ring to the tubing outer surface and the strips were also prone to being rubbed or tom off during delivery in a deviated wellbore.
What is needed and addressed by the present invention is a centralizer that can be quickly installed without having the tubulars delivered to a specialized facility. The centralizer can then be fitted to the tubular and function to centralize in a deviated wellbore for cementing while not providing meaningful resistance to expansion of the tubular. The centralizer can incorporate a mechanism to allow expansion and retain a secured initial position for delivery and cementing and maintain a secured post expanded position. These and other features of the present invention will become more apparent to those skilled in the art from a review of the description of the preferred embodiment and the drawings associated with it, recognizing that the full scope of the invention is illustrated in the appended claims.